This invention relates to the stabilization of modified celluloses in aqueous salt solutions or brines; such stabilization is effective at moderate and high temperatures. This invention is also directed to brine fluids containing modified cellulose, especially hydroxyethyl cellulose as viscosifying agents wherein stabilization of the modified cellulose to high temperature has been accomplished. This invention also deals with certain techniques employed in oil and gas production notably well completion and "workover", which employ such stabilized, viscosified brines.
An important step in the production of oil and gas from wells is the "completion" of a drilled well. Such completion may comprise numerous subtasks including perforation, cementing, acidizing, fracturing, and others. Those skilled in the art will recognize that one important task is the removal of well debris through the use of a completion fluid. The irrigation of a drilled well with a completion fluid has a salutary effect of washing or floating out various forms of drilling or other debris which could otherwise clog the petroleum producing strata and impede delivery of oil or gas to the production site for recovery. A similar type of fluid is employed during workover of an established well. Workover is a term generic to a process for revitalizing a well to improve its productive performance. Fluid removal of debris following such a workover is usually required; fluids for this purpose share many requirements with completion fluids. Such fluids will be denominated generically hereinafter as "clear brines".
Those skilled in the art will appreciate that numerous requirements may be imposed upon completion and workover fluids (clear brines) depending upon the exigencies of oil field operations. The requirements which are of chief relevance to the present invention are the requirements of specific gravity or density and of viscosity. It will be understood that conditions "downhole" of a petroleum well are nearly always substantially different from standard temperature and pressure. In general, such locations are under elevated hydrostatic pressure from surrounding strata and experience elevated temperatures. Temperatures in excess of 225.degree.-250.degree. F. are common and much higher temperatures are not uncommon. As wells are sunk to greater and greater depths, higher temperatures and pressures will be experienced.
It has long been appreciated that clear brines for completion and workover must, in general, have specific gravities or densities higher than that of water. Thus, such fluids commonly comprise aqueous solutions of inorganic salts resulting in densities higher than water's 8.3 pounds per gallon. For example, saturated solutions of potassium chloride, sodium chloride, and calcium chloride have densities of 9.7, 10.0, and 11.6 pounds per gallon respectively; such brines are frequently employed in completion and workover operations. Such elevated densities are needed in order to maintain a hydrodynamic equilibrium between the downhole pressure and the clear brine completion or workover fluid. Without aid of this high density, the equipment employed for fluid circulation would be liable to failure. Such increased density also aids in the suspension of debris particles for transport out of the well.
The second requirement for clear brines which is of importance in the context of the present invention is that of viscosity. It is known that such brines should possess viscosities which facilitate the suspension and transport of debris particles. Accordingly, viscosifiers are generally added to such brines. Such viscosifiers have included guar, xanthan, and other gums, modified celluloses, especially hydroxyethyl cellulose (HEC), and biologically modified saccharides such as "XC" polymer and others. The preferred materials for such viscosifiers are the modified celluloses, especially HEC.
A major problem with the employment of HEC and brines used at high temperatures has been a relatively rapid breakdown of viscosity of the solutions thus used. This breakdown, which is believed to be caused by depolymerization of the cellulosic structure or by other modifications thereof, has diminished the effectiveness of completion and workover applications employing such viscosified brines. The breakdown in viscosity is most dramatically effected by changes in downhole temperature, becoming serious above about 225.degree. F. and critical above about 250.degree. F.
While the maintenance of viscosity in such clear brines is desirable during completion or workover operations, following termination of such procedures, it is frequently desirable to "break" the viscosity intentionally. It is known that acidification of HEC-modified brines will cause hydrolysis of the HEC and the diminution in the viscosity thereof. It is desired to maintain the ability to break the HEC-modified brines with acid or otherwise when desired while avoiding degradation in viscosity during use for completion or workover.
Accordingly, it is an object of this invention to provide a method for the stabilization of modified cellulose in clear brines. Another object is to provide clear brines viscosified with modified cellulose having viscosity which breaks down at a decreased rate at temperatures above about 225.degree. F. Still another object of this invention is to facilitate improved efficiency in completion and workover operations at elevated temperatures. Another object is to provide clear brines which may be caused to lose a substantial portion of viscosity upon acidification or other selected treatment. These and other objects are obtained to the use of the novel processes and compositions of this invention.